Integrated embedded processor based laser spectroscopic sensor

ABSTRACT

A novel low-power and compact laser spectroscopic sensor is described herein. Embodiments of the disclosed sensor utilize state-of-the-art microprocessors and digital processing techniques to reduce power consumption and integrate functions into a small device. In particular, novel software methods are disclosed which allow the use of low-power microprocessors which draw no more than about 0.02 W of power. Such low-power enables long battery life and allows embodiments of the sensor to be used in portable applications. In addition, the system architecture and methods described in this disclosure allow a single integrated embedded processor to control all the subsystems necessary for a laser spectroscopic sensor further reducing sensor size and power consumption. In addition, a power efficient method of calibrating a photoacoustic laser spectroscopic sensor is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a filing under 35 U.S.C. §371 of InternationalApplication No. PCT/US2006/039282 filed Oct. 6, 2006 by Stephen So, etal. and entitled “Integrated Embedded Processor Based LaserSpectroscopic Sensor,” which claims priority to U.S. Provisional PatentApplication 60/824,843 filed Sep. 7, 2006 by Stephen So, et al. andentitled “Integrated Single Embedded Processor Based Laser SpectroscopicSensor,” both of which are incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of laser basedspectroscopy. More specifically, the invention relates to an integrated,low-power, laser spectroscopic sensor.

BACKGROUND

There are several widely used laser spectroscopy techniques whichinvolve measurement of absorption of the laser light within a sample. Inorder to achieve high sensitivity in many cases the measurement signalshave to be extracted from noisy background. This usually involves phasesensitive detection of the modulated laser radiation. PAS (photoacousticspectroscopy) is an analytical method that involves stimulating a samplewith modulated light and detecting the resulting sound waves emanatingfrom the sample. A photoacoustic measurement can be made as follows.First, light is used to excite molecules within a sample. Suchexcitation can include, for example, absorption of the light by themolecule to change an energy state of the molecule. As a result, theenergized molecule enters an excited state. Optical excitation isfollowed by energy transfer processes (relaxation) from the initiallyexcited molecular energy level to other degrees of freedom, inparticular translational motion of the fluid molecules. During suchrelaxation, heat, light, volume changes and other forms of energy candissipate into the environment surrounding the molecule. Such forms ofenergy cause expansion or contraction of materials within theenvironment. As the materials expand or contract, sound waves aregenerated.

In order to produce sound waves, or photoacoustic signals, the light ismodulated at a specific acoustically resonant modulation frequency f(having a modulation period 1/f), sometimes also referred to herein asω. The sample environment can be enclosed and may be constructed toresonate at the modulation frequency. An acoustic detector mounted inacoustic communication with the sample environment can detect changesoccurring as a result of the modulated light excitation of the sample.Because the amount of environmental change associated with the absorbedenergy is proportional to the concentration of the absorbing molecules,the photoacoustic signal can be used for concentration measurements.

In typical PAS, a resonant acoustic cavity or sample cell is used toisolate and amplify sound wave signals, thereby increasing sensitivityof detection. The light intensity or wavelength is modulated at afrequency, f. The absorbed energy is accumulated in the acoustic mode ofthe sample cell during oscillation periods. Quartz enhancedphotoacoustic laser spectroscopy (QEPAS) has been found to be highlysensitive and selective technique for the detection of gasconcentrations at the parts-per-billion (ppb) and parts-per-trillion(ppt) level. Because of its sensitivity, QEPAS may be useful in manydifferent applications. A number of applications, however, require anultra-compact footprint i.e. small size, and low power consumption.

Currently, systems are based on modular architectures with an externallymounted laser source, separate power and thermal controllers,environmental transducers, and/or separate processing hardware andsoftware. Such systems require human feedback to operate and may not beconsidered to be truly integrated. In addition, at present, sensorstypically utilize separate sub-system controllers running independently.Systems with independent sub-systems, as such, cannot be consideredfully integrated. Because each sub-system requires a respectivecontroller, present systems are bulky, expensive, and requireimpractical amounts of electrical power.

Consequently, there is a need for a fully integrated trace-gas sensorplatform which is low cost, compact, and power efficient.

SUMMARY

A novel low-power and compact laser spectroscopic sensor is describedherein. Embodiments of the disclosed sensor utilize state-of-the-artmicroprocessors and digital processing techniques to reduce powerconsumption and integrate functions into a small device. In particular,novel software methods are disclosed which allow the use of low-powermicroprocessors which draw no more than about 0.02 W of power. Low powerconsumption enables long battery life and allows embodiments of thesensor to be used in portable applications. In addition, the systemarchitecture and methods described in this disclosure allow a singleintegrated embedded processor to control all the subsystems necessaryfor a laser based spectroscopic sensor further reducing sensor size andpower consumption.

These and other needs in the art are addressed in one embodiment by alaser spectroscopic sensor for detecting a compound comprising adetector capable of transmitting a signal in response to absorption oflight by the compound. The laser spectroscopic sensor further comprisesa light source having a modulation frequency. The light sourceintroduces a beam of light to said acoustic detector. The laserspectroscopic sensor also comprises a microprocessor coupled to saidlight source and said acoustic detector. In addition, the laserspectroscopic sensor comprises software executable on saidmicroprocessor. The software causes said microprocessor to control thetemperature, wavelength, and modulation frequency of said light source.The software also causes the microprocessor to acquire and process datafrom said detector. Additionally, the software causes the microprocessorto generate a first waveform having a first frequency. The firstwaveform is divisible into a plurality of different waveforms and eachwaveform has a frequency which is a multiple of the modulation frequencyof said light source.

In a further embodiment, the detector is an acoustic detector having aresonant frequency. The software executable on the microprocessor causesthe microprocessor to iteratively generate the first waveform to tunethe modulation frequency of said light source to the resonant frequencyof said acoustic detector.

In another embodiment, a method for calibrating a laser spectroscopicsensor comprising an acoustic detector, a lock-in amplifier, and a lightsource having a modulation frequency, comprises (a) generating a firstwaveform having a first frequency. The first waveform has a firstfrequency greater than twice the modulation frequency of said lightsource. Moreover, the method comprises (b) forming a plurality ofsynchronized waveforms from the first waveform, wherein eachsynchronized waveform is different. Furthermore, the method comprises(c) tuning the reference frequency of the first lock-in amplifier andthe modulation frequency of the light source with the plurality ofsynchronized waveforms. Additionally, the method comprises (d)determining whether the modulation frequency of the light source istuned to a harmonic resonant frequency of the acoustic detector. Ifmodulation frequency of the light source is not tuned to a harmonicresonant frequency of the acoustic detector, the method comprises (e)adjusting the first frequency. The method further comprises f) repeatingsteps (a) through (e) until the modulation frequency of the light sourceis tuned to the resonant frequency of the acoustic detector so as tocalibrate the laser spectroscopic sensor.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings inwhich:

FIG. 1 illustrates an embodiment of an integrated laser spectroscopicsensor.

FIG. 2 illustrates an embodiment of a method for calibrating a laserspectroscopic sensor.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. This document does not intendto distinguish between components that differ in name but not function.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ”. Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect electrical connection. Thus, if a first device couples to asecond device, that connection may be through a direct electricalconnection, or through an indirect electrical connection via otherdevices and connections.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of a photoacoustic laser sensor. Themethods disclosed herein here may be extended to any modulation basedlaser spectroscopy such as photodetector-based laser spectroscopy. In anembodiment, a laser spectroscopic sensor is configured to apply amodulated light signal to a sample and to detect the resulting acousticsignal using a phase-locked detector such as a lock-in amplifier. By wayof example, reference is made to FIG. 1, in which a laser spectroscopicsensor 100 comprises a light source 112 configured to emit a beam ofradiation into a sample cell 118. According to at least one embodiment,all elements of laser spectroscopic sensor 100 are mounted on a smallfootprint circuit board. Light source 112 typically comprises a laser.However, any light source capable of emitting a modulated beam of lightmay be used. In a preferred embodiment, light source is a near infra-redsemiconductor diode laser. Other examples of suitable lasers that may beused include without limitation, lead salt diode lasers, quantum cascadeand interband cascade lasers, fiber lasers, solid-state lasers, othersemiconductor lasers, or gas lasers. Filters (not shown) may be providedbetween light source 112 and sample cell 118 if desired.

Laser spectroscopic sensor 100 generally comprises a sample cell 118which encloses a detector 120 and contains a sample compound ofinterest. However, in some embodiments, laser spectroscopic sensor 100comprises detector 120 without sample cell 118. Sample cell 118 may be amultipass cell or any other absorption chamber if using anon-photoacoustic method. Sample cell 118 can comprise a number ofmaterials known to persons of ordinary skill in the art, and preferablycomprises a sample compound substantially transparent to thewavelength(s) of light emanating from light source 112. Preferred samplecompounds for sample cell 118 will accordingly vary depending on thewavelengths of light utilized in the spectroscopic apparatus. Samplecompound may be a fluid or a gas and may substantially fill sample cell118. Sample compound can, for example, comprise a gas stream in which itis desired to detect the presence of a contaminant gas or impurity.Thus, in some embodiments, sample cell 118 includes a pump (not shown)to adjust flow of a sample into sample cell 118. In an embodiment, apressure sensor 121 such as a resistive bridge pressure transducer iscoupled to sample cell 118 to measure the pressure within sample cell118. In addition, other sensors may be coupled to sample cell 118 tomeasure temperature, pH, etc.

The detector 120 may be mounted within sample cell 118 and in acousticcommunication with a sample. Detector 120 preferably comprises anacoustic transducer such as, for example, a piezoelectric element or amicrophone and is mounted such that a sample compound is providedbetween a surface of detector 120 and sample cell 118. In the embodimentshown, detector 120 comprises a quartz tuning fork. However, thedetector 120 may comprise any suitable piezoelectric or resonant crystalmaterial. In alternative embodiments (not shown), detector 120 can beany type of detector (e.g. photodetector) capable of detecting theabsorption of light by a compound. Detector 120 may be mounted on theinside or outside wall of sample cell 118. Detector 120 is typicallyremovably mounted into sample cell 118. In an embodiment, detector 120additionally comprises a resonator (not shown) to further amplify theacoustic signal from detector 120. The resonator is typicallycylindrical in configuration, but may comprise any suitable geometry.Typically, sample cell 118 also comprises a collimator 127 to focus thebeam of light to detector 120.

Detector 120 is in electrical communication with a preamplifier 122,which is preferably in electrical communication with a first lock-inamplifier 152. Preamplifier 122 is used to convert and amplify thesignal from detector 120 to the appropriate level for detection by firstlock-in amplifier 152. In an embodiment, preamplifier 122 is atransimpedance preamplifier. Lock-in amplifiers are well-known in theart and typically comprise a low pass filter and a phase-sensitivedetector. Both lock-in amplifiers 148 and 152 are preferably integratedinto the laser spectroscopic sensor. As such, any lock-in amplifiers orother demodulation devices known in the art may be used with embodimentsof the sensor. First lock-in amplifier 152 is coupled to microprocessor124. In certain embodiments, microprocessor 124 processes the amplifiedsignal from first lock-in amplifier 152 as described in further detailbelow.

In a further embodiment, laser spectroscopic sensor 100 comprises areference cell 144. Reference cell 144 generally contains a referenceconcentration of the target compound of interest. Typically, aphotodetector 146 is coupled to reference cell 144. However, any devicemay be coupled to reference cell 144 to detect absorption. Photodetector146 senses the absorption by the reference concentration in referencecell 144. Photodetector 146 is also in electrical communication with asecond lock-in amplifier 148. In some embodiments, a preamplifier (notshown) may be disposed between photodetector 146 and second lock-inamplifier 148. Both first and second lock-in amplifiers 152, 148 arepreferably dual phase lock-in amplifiers.

A beam splitter 126 may be included in the sensor and can be configuredto facilitate division of the through beam of light. Beam splitter 126splits the light signal into a first and second beam, where first beamis directed at sample cell and second beam is directed at referencecell. In further embodiments, beam splitter 126 splits beam into morethan two beams. Beam splitter 126 may be any suitable device known inthe art.

In a preferred embodiment, the sensor 100 comprises a singlemicroprocessor 124 such as a low-power digital signal processor. Forexample, the microprocessor 124 may be a MSP430-class DSP processorcommercially available from Texas Instruments, Inc. However, anysuitable microprocessors may be used with the laser spectroscopicsensor. Other examples of suitable processors include withoutlimitation, field programmable gate arrays, microcontrollers,programmable logic devices, application specific integrated circuits andthe like. The microprocessor 124 controls all the sub-systems orfunctions of the laser spectroscopic sensor 100 including withoutlimitation, diode laser temperature control, diode laser currentcontrol, sample gas temperature, sample gas pressure, signalconditioners, waveform generation, etc. It is preferred that allsub-system controls of the laser spectroscopic sensor are integrated ona single microprocessor. Integration of all controls in a singlemicroprocessor eliminates the need for a bulky external controllingdevice such as a computer, or external control hierarchy. In addition,using a single microprocessor 124 consumes less power and reducescomplexity in the laser spectroscopic sensor 100. However, it iscontemplated that additional embodiments of the laser spectroscopicsensor 100 may utilize more than one microprocessor.

In embodiments, microprocessor 124 includes memory 191. Memory 191 maycomprise volatile (e.g., random access memory) and/or non-volatilememory (e.g., read only memory (ROM), electrically-erasable programmableROM (EEPROM), Flash memory, etc.). In a preferred embodiment, memory 191is flash memory. Memory 191 may be used to store data or code (e.g.,software, discussed below) that is executed by the microprocessor 124.The executable code may be executed directly from the non-volatilememory or copied to the volatile memory for execution therefrom. Laserspectroscopic sensor 100 may also include memory external tomicroprocessor 124. This external memory is generally coupled tomicroprocessor 124 and may comprise either volatile or non-volatilememory.

In another embodiment, a plurality of frequency dividers (not shown) arecoupled to microprocessor 124. As defined herein, a frequency divider isany module or circuit which divides a waveform or signal into a lowerfrequency waveform or signal. In a preferred embodiment, the pluralityof frequency dividers are asynchronous counters. However, the frequencydividers may comprise other types of frequency dividers known in art.The frequency dividers are used to divide the waveform generated bymicroprocessor 124 as will be described in more detail below.

It is contemplated that many sensing devices or modules may be inelectrical communication with microprocessor 124 to form multiplecontrol loops. For example, in further embodiments, a current controllermodule 161 and a thermoelectric module 163 are in electricalcommunication with microprocessor 124. Current controller module 161 andthermoelectric module 163 are also in electrical communication withlight source 112. Microprocessor 124 controls current controller 161 toadjust current of light source in response to changes in resonantfrequency of detector. Current controller module 161 is also responsiblefor adjusting the central wavelength and the wavelength modulation oflight source 112. Thermoelectric module 163 controls the temperature oflight source since temperature affects the frequency of the light signalemitted from light source. In certain embodiments, a temperature sensor(not shown) is coupled to light source 112 which transmits temperaturedata to microprocessor 124.

According to one embodiment, the microprocessor draw less than about0.05 W, more preferably less than about 0.02 W. Low power consumption isan important aspect of the laser spectroscopic sensor 100, as the lesspower is used or drawn from microprocessor, the longer the sensor may beused in portable applications. Thus, in preferred embodiments, thesensor 100 is powered by a battery such as a lithium ion battery (notshown).

Microprocessor 124 may be coupled to a variety of differentcommunication devices (not shown). In an embodiment, microprocessor 124is coupled to an RF or wireless antenna. Alternatively, microprocessor124 is coupled to a wireless chip. In addition, microprocessor 124 maybe coupled to a communications port such a Universal Serial Bus Port, aserial port, a parallel port, Firewire port, etc. In another embodiment,the laser spectroscopic sensor 100 includes input devices allowing auser to input parameters for using laser spectroscopic sensor 100. Theinput devices may be coupled to microprocessor 124 to programmicroprocessor or adjust laser spectroscopic sensor 100 parameters.Example of input devices include without limitation, keypads, jumpers,touch sensors, and buttons.

In a preferred embodiment, the laser spectroscopic sensor 100 includingall of its individual modules (e.g. detector, microprocessor, lightsource, etc.) is mounted or is capable of fitting on a single circuitboard. Thus, another novel feature of the disclosed sensor 100 is itsultra-compact size. It is envisioned that embodiments of laserspectroscopic sensor 100 will be no larger than a personal digitalassistant or a portable MP3 player, thus, allowing placement of manysuch sensors 100 in remote locations. In general, laser spectroscopicsensor 100 including light source 112, microprocessor 124, and all otherelectronics consumes no more than 5 W of power, preferably no more than1 W of power.

In operation, a beam of light is generated by light source 112 accordingto a signal from microprocessor 124 and is passed through sample cell118 to excite the molecules within the sample compound in sample cell118. The microprocessor 124 generally provides a reference electricalsignal in the form of a sine wave or rectangular wave synchronized tothe light modulation. Nonradiative decay or molecular rearrangementscause expansions and/or contractions of a material within sample cell118 to generate acoustic waves passing from sample to detector 120. Inphotoacoustic embodiments, detector 120 detects the resulting acousticwaves and passes signals corresponding to, for example, gas pressurechanges in the acoustic waves to first lock-in amplifier 122.Alternatively, detector 120 is a photodetector which measures theintensity of the beam of light after absorption by the sample compound.The change in intensity is proportional to the concentration of thetarget compound in the sample.

Both first and second lock-in amplifiers 152, 148 generally comprise twochannels and produces two outputs (DC voltage levels, X and Y)corresponding to in-phase and quadrature (e.g. 90 degrees), componentsof the detector signal with respect to the reference signal. However,the lock-in amplifiers 152, 148 may also be single channel amplifiers.The signal from first lock-in amplifier 152 is then sent tomicroprocessor 124 for acquisition and processing. An output device maybe coupled to sensor 100 (not shown) and be configured to convertinformation obtained from microprocessor 124 to, for example, agraphical or numerical display.

As mentioned above, beam splitter 126 divides the beam of light into afirst beam and second beam, in which second beam is directed atreference cell 144. Reference cell 144 contains a referenceconcentration of the target compound to be measured. Photodetector 146provides a signal at the wavelength at which the target compound absorbsthe light. The signal is relayed through second lock-in amplifier todetect the wavelength error. The wavelength error measurement is thensent to microprocessor 124. Microprocessor 124 performs a computation onthe wavelength error signal, and sends this error factor to currentcontroller 161 to adjust the wavelength of light source 112. Thisfeedback loop ensures that the light source 112 is emitting light at theappropriate wavelength corresponding to the absorption line of thetarget compound. This wavelength control is also known as“line-locking.” In additional embodiments, microprocessor controls thewavelength modulation of light source 112 via current controller module161.

In a further embodiment, software executable on microprocessor 124allows for data acquisition and processing from detector 120. Asmicroprocessor 124 receives a signal from detector 120 via first lock-inamplifier 152, the software instructs microprocessor to store the signallevel in memory 191. The software also enables microprocessor 124 tocalculate the concentration of the target compound in the sample usingthe acquired data (i.e. signal level). Furthermore, the software mayinstruct microprocessor to send the calculated concentration to anoutput device through any communications devices coupled tomicroprocessor 124 such as a USB port or wireless chip.

In embodiments utilizing an acoustic detector, software executable onthe microprocessor 124 matches the modulation frequency of the lightsource 112 and the lock-in amplifier frequencies with the resonantfrequency of the detector 120. The resonant frequency of the detector120 is variable because of changes in temperature and pressure in thesample chamber 118. In order to maximize the signal from the detector120, the modulation frequency of the light source 112 is tuned to matchthe resonant frequency of the detector 120. In addition, the lock-inamplifiers 152, 148 are tuned or programmed to the detector resonantfrequency in order to amplify only signals at the detector's resonantfrequency. A power-efficient and novel method for performing theaforementioned calibration is described below.

As shown in FIG. 2, in a preferred embodiment, the software causes themicroprocessor 224 to periodically calibrate or tune the modulationfrequency of the light source to the resonant frequency of an acousticdetector 220. In an embodiment, the software causes the microprocessorto check the resonant frequency every 1 minute to 20 minutes, preferably10 minutes. However, the period between frequency calibrations ortunings may be any suitable time period. In an embodiment, the softwarecauses the microprocessor to calibrate the resonant frequencycontinuously. Referring now to FIG. 2, to begin the calibration process,the microprocessor synthesizes or generates a first waveform that isdivisible into a plurality of different waveforms at lower frequenciesin block 210. Furthermore, the software may cause the microprocessor toshut off light source during the calibration or tuning process. In apreferred embodiment, microprocessor 224 generates a waveform that isdivisible into 5 lower frequency waveforms. Typically, f is initiallythe modulation frequency of the light source from the previouscalibration. According to at least one embodiment, the first waveformhas a frequency of 12f. However, waveforms of any suitable frequency maybe generated.

In at least one embodiment, the software causes the microprocessor 224to generate the first waveform using a direct digital synthesisalgorithm (DDS). However, any suitable methods may be used to synthesizethe waveform such as programmable and controlled oscillators,direct-analog synthesis or indirect synthesis. The generated waveform issent to a plurality of frequency dividers to divide the first waveforminto a plurality of synchronized waveforms. That is, the plurality ofwaveforms may be formed in parallel (i.e. simultaneously) or with someother timing pattern. As mentioned above, the plurality of frequencydividers may be a plurality of digital counters. Other frequencydividers may also be used. Preferably, the 12 f waveform is sent to 5different digital counters which divide it into 5 respective waveformsin block 211. In an embodiment, each of the 5 waveforms has one of thefollowing frequencies: f, 2f, 2f+90 degrees, 3f, 3f+90 degrees, where fis the modulation frequency of light source 212. Alternatively, thefirst waveform may be divided into any waveform having a frequency thatis a multiple of f (i.e. 2f, 3f, 4f, 5f, etc.).

The 2 f and 2f+90 degree waveforms are sent as reference signals to thereference and quadrature channels of the first lock-in amplifier 252,respectively. In addition, the 2f waveform signal may be sent todetector 220 to excite the acoustic detector 220 if laser excitationdoes not provide a strong enough signal. The 3f and 3f+90 degreewaveforms are sent to the reference and quadrature channels of secondlock-in amplifier 248, respectively. The f waveform is sent to the lightsource current controller where the modulation frequency is adjusted ortuned to match the detector resonant frequency. Therefore, the softwareexecutable on microprocessor 224 is optimized such that the onlyfunction for frequency calibration performed by the microcontroller 224is to iteratively generate a first waveform divisible into the 5specific waveforms. Accordingly, a novel aspect of the software is thata plurality of synchronized waveforms may be generated with minimalprocessing and power draw by microprocessor 224.

A preamplifier 122 converts the signal from the detector to sufficientvoltage levels for the first lock-in amplifier 252 to detect. Thatsignal is connected to the first lock-in amplifier 252. First lock-inamplifier 252 and light source 212 must be tuned to the resonantfrequency of the detector in order to generate and amplify the signalfrom acoustic detector 220. If first lock-in amplifier 252 and lightsource 212 are not provided with the correct reference frequency, thesignal from acoustic detector 220 will not be maximized. Ifmicroprocessor 224 determines that the signal from first lock-inamplifier 252 has not reached a maximum value in block 213,microprocessor 224 iterates another frequency in block 215 and generatesanother first waveform at this different frequency. This waveform iscontinuously divided by digital counters and sent to each respectivemodule i.e. light source, lock-in amplifiers, etc.

The software causes the microprocessor 224 to continue iterating andgenerating new waveforms with different frequencies until microprocessor224 determines that the signal from first lock-in amplifier 252 hasreached a maximum value. In an embodiment, the software utilizes abinary search algorithm to determine whether the signal from lock-inamplifier 252 is maximized. Without being limited by theory, it isbelieved that once the signal from lock-in amplifier 252 is maximizedthe modulation frequency of light source 112 is matched with theresonant frequency of the acoustic detector 220. Once an amplifiedsignal from the first lock-in amplifier 252 at the specific resonantfrequency of the acoustic detector is detected by microprocessor 224,the software halts the tuning or calibration process. If the signal tonoise ratio is high enough, the modulation frequency may itself bemodulated and a lock-in amplifier may be used to lock in the resonantfrequency.

Referring back to FIG. 1, in embodiments of laser spectroscopic sensor100 utilizing a photodetector (not shown), the frequency of the firstwaveform generally is not iterated or adjusted. Instead, themicroprocessor 124 is programmed to repeatedly generate a first waveformat a constant first frequency. For example, in embodiments of sensor 100having first and second lock-in amplifiers 152, 148 and a photodetector,the first waveform is still divided into a plurality of differentwaveforms using a plurality of frequency dividers. Each waveform fromthe plurality of frequency dividers is sent to the respective channelsof the lock-in amplifiers as well as light source control. However, thefrequency of each of these waveforms does not change over time becausethe frequency of the first waveform remains constant. As a result, thedisclosed techniques may increase the power efficiency forphotodetection embodiments of the sensor 100 as only one waveform at asingle frequency needs to be generated by the microprocessor 124.Nevertheless, it is contemplated that the calibration method foracoustic detectors described above may also be used with a photodetectorif desired.

The software executable on microprocessor may further utilize pulsewidth modulation (PWM) to control individual sub-systems of laserspectroscopic sensor 100. In another embodiment, software executable onmicroprocessor causes the microprocessor to automatically perform PWMpower conversion from a power supply for the light source or to use PWMto heat and cool the light source.

The cost effectiveness and low-power utilization of the disclosed sensor100 allows for the application of many sensors as nodes in a wirelesssensor network. The sensors may be integrated into common handhelddevices with other functionality (e.g., cell phones or personal digitalassistants (PDAs)) which may be used in self-diagnostic healthapplications or personal air quality control (helpful in urban orindustrial environments).

A wireless network on the scale of hundreds of nodes would enableapplications such as source localization for fire detection, or widearea monitoring for environmental applications. These sensors may alsobe capable of utilizing environmentally friendly energy sources (e.g.solar, wind, vibration), and work together to determine optimum dutycycles for each member of the network.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A method comprising: generating a first waveform at a firstmodulation frequency using a direct digital synthesis (DDS) algorithm;and dividing the first waveform into a plurality of second waveforms ata plurality of second modulation frequencies using a plurality offrequency dividers, wherein the second modulation frequencies comprise alight source modulation frequency that is used to control the frequencyof a light source, a first in-phase modulation frequency and a firstquadrature modulation frequency that are used to detect absorption of afirst portion of the light source by a sample compound, and a secondin-phase modulation frequency and a second quadrature modulationfrequency that are used to detect absorption of a second portion of thelight source by a reference concentration of the sample compound, andwherein a frequency initially used to modulate the light source is equalto about f, wherein the first modulation frequency is equal to orgreater than about 12f, wherein the light source modulation frequency isequal to about f, wherein additional timing waveforms comprise the firstin-phase modulation frequency, the first quadrature modulationfrequency, the second in-phase modulation frequency, and the secondquadrature modulation frequency, wherein the first in-phase modulationfrequency is equal to about 2f, wherein the first quadrature modulationfrequency is equal to about 2f+90 degrees, wherein the second in-phasemodulation frequency is equal to about 3f, and wherein the secondquadrature modulation frequency is equal to about 3f+90 degrees.
 2. Themethod of claim 1, wherein the first waveform is generated using asoftware that is executed on a processor.
 3. The method of claim 2,wherein the processor is a fixed point digital signal processor.
 4. Themethod of claim 3, wherein the frequency dividers are hardware timersthat are part of the processor.
 5. The method of claim 4 furthercomprising detecting at least one of the second waveforms using alock-in amplifier comprising a filter and a phase sensitive detector. 6.The method of claim 5, wherein a transimpedence preamplifier is used todetect the second waveform, wherein the transimpedence preamplifier iscoupled to the lock-in amplifier.
 7. The method of claim 1, whereindividing the first waveform into the second waveforms is repeated untila signal level for detecting absorption of the first portion of thelight source by the sample compound reaches a limit.
 8. An apparatuscomprising: a light source; and a processor configured to implement amethod comprising: dividing a first waveform into at least a secondwaveform having a lower frequency; and filtering the second waveform tocreate a sinusoid; attenuating the sinusoid using control signals fromthe processor; and controlling a current fed to the light source usingthe attenuated sinusoid, thereby controlling a modulation of a lightemitted from the light source; a detector; an amplifier coupled to thedetector and the processor, wherein the amplifier receives a signal fromthe detector, performs phase sensitive detection of the signal, andpasses the signal to the processor and wherein the processor, the lightsource, and the detector are mounted on a circuit board, and wherein thecircuit board consumes a power less than about five watts (W); awireless transmitter coupled to the processor; and a power sourcecoupled to the circuit board, wherein the processor produces dataassociated with emission or uptake of a chemical, and wherein thewireless transmitter transmits the data to a central location.
 9. Theapparatus of claim 8, wherein the detector is an acoustic detector. 10.The apparatus of claim 8, wherein the detector comprises an absorptioncell and a photodetector.